Any high-end luxury sedan should deliver refinement in spades; for the most part, leading players like the Mercedes S-Class and BMW 7 Series do just that. Pitted against such competition, the Audi A8 hasn’t made much of a sales dent. Now, Audi has redesigned its flagship for 2011, and the results suggest a concerted effort to build something unabashedly different.

Indeed, the Audi A8 is distinct, with a refreshingly progressive cabin, but the driving experience lacks polish — which is why Mercedes and BMW will remain the mainstream choices.

The full-size A8 sedan comes with a V-8 or W-12 engine in regular- and extended-wheelbase versions. All-wheel drive is standard. I drove the regular-wheelbase A8 with a V-8 engine.

Outside & InAudi introduced its plunging grille on the prior A8, and now every other Audi has it. If this A8 portends Grille 2.0, some may wish for a reboot. The massive opening has eight slats that stretch from hood to ground. If your state requires one, the front license plate will have to hang somewhere in the chromed abyss below Audi’s four-ring logo. While the new face drew criticism from a few editors here, I didn’t mind it as much as I did our test car’s headlights, whose scaly LEDs look downright reptilian.

The extended-wheelbase A8 L adds 5.2 inches of length. Most of the extra space goes to the rear seats, which gain 4.2 inches of legroom and a slew of optional features — climate controls and seat massagers for the outboard passengers, a mini refrigerator and more. Even without those extras, our regular-wheelbase tester had a comfortable backseat, with ample legroom and excellent thigh support for my 5-foot-11 frame.

Our test car’s front seats had exceptional adjustment range, and their memory settings remember the smorgasbord of power adjustments available, including lumbar and side-bolster support — a rarity. Thigh support up front is excellent, but over a two-hour drive I found the backrests a bit stiff.

Cabin materials are quite good, with high-grade leather upholstery and premium surfaces nearly everywhere, prompting fellow editor Mike Hanley to declare that Audi has raised the bar yet again. Audi’s Multi Media Interface sports a new thumb pad, whose capabilities include scrolling the navigation system’s map. You can also use your finger to trace letters and numbers on the pad to input a navigation destination. Located on the center console, it doesn’t require you to look down to operate. Destination inputs are fairly intuitive, but in heavy urban areas the map shows woefully few street labels. Trace-scrolling, too, is frustratingly laborious; the scrolling knobs in the S-Class and many cheaper cars are a better approach.

Trunk volume is 13.2 cubic feet, which is smaller than the trunks in some compact cars. Here, it’s unacceptable; the S-Class and Lexus LS have 16 cubic feet or more. A center pass-through for skis or other long items is optional on both the A8 and the A8 L.

Going & StoppingThe A8 moves out well enough, but it falls short of the effortless punch of the Mercedes S550 or BMW 750i. Load the car up with passengers and cargo, and the 4.2-liter V-8 can feel modest. Still, at higher revs it belts out a pleasing, high-pitched whine, and Audi’s all-wheel drive sends enough power to the rear to make the car feel like a proper sports sedan. Drive solo, and the car should move plenty quick for all but the most power-hungry driver.

Later this year, said lead-foots will be able to move up to the 6.3-liter W-12, which is similar to a V-12 but with offset cylinder banks. It makes 500 horsepower versus the V-8’s 372 hp, and it comes only on the extended-length A8 L. Audi pegs its zero-to-60 mph acceleration at 4.9 seconds, versus the V-8’s 5.7 seconds. Both engines use an eight-speed automatic transmission, whose short gears make for rapid-fire upshifts. It’s a smooth operator with the V-8, but it could use better kickdown response. Turn a corner at lower speeds and attempt to accelerate, and the automatic sometimes dallies in a higher gear. With a comparatively modest 328 pounds-feet of torque — significantly less than its competitors’ V-8 ratings — the A8’s 4.2-liter engine doesn’t have quite the low-end thrust to cover for a lazy transmission. It’s not the worst transmission in the class, but others — particularly the Jaguar XJ’s six-speed auto — are far better.

Four-wheel-disc antilock brakes are standard. Like hybrids and some conventional cars, the A8 employs regenerative braking to aid its overall fuel efficiency — which, incidentally, is rated an exemplary 21 mpg in combined city/highway driving. Alas, as with most instances of regenerative braking, you pay a price in performance. The pedal just isn’t that linear: There’s an inch or so of limp pedal travel at first, and it’s too grabby at the end to manage smooth stops easily.

Ride & HandlingPushed hard, the A8 holds its own, but it doesn’t beg to be driven hard like the XJ does. An optional sport differential sends more power to the outside rear wheel in corners. Our car had it, and dialed to its most aggressive setting the differential gives the tail a drift-happy demeanor, and the steering’s sharp turn-in precision makes rotating the car enjoyable. But in hard corners the A8 exhibits a lot of body roll — perplexing, given its firm-ish ride — and the modest drivetrain doesn’t give you much confidence when accelerating out.

The A8’s Sport Package includes firmer calibrations for the car’s standard adaptive suspension; also part of the package are 20-inch wheels and thin P265/40R20 tires. Thus equipped, our test car rode a bit on the firmer side, even in the softest of the suspension’s three settings. It’s not likely to prove too objectionable for many shoppers — the LS is similarly firm, and the XJ is even firmer — but a car this plush ought to ride on clouds. The S-Class nails it, and the 7 Series comes very close. Indeed, a non-Sport A8 might fare better. If you test one back-to-back with the Sport, click the link at the end of this review to email me your assessment.

Included in the Sport Package is Audi’s Dynamic Steering, which — similar to BMW’s Active Steering — can vary steering ratio and assist. Like the suspension, the steering offers three settings. Unfortunately, none of them are very good. Characteristic of many big luxury cars, the wheel turns with a light touch at low speeds. The A8 abruptly dials back the power assist as you speed up, but the transition isn’t very smooth: It often takes a second to catch up, with awkward moments of low assist if you slow down too quickly.

Safety, Features & PricingAs of this writing, the A8 hasn’t been crash-tested. Standard safety features include 10 airbags, antilock brakes and an electronic stability system. Click here for a full list, and here to see our full evaluation of the A8’s child-seat accommodations. Safety options include a forward collision warning system with automatic braking, as well as blind spot and lane departure warning systems.

The V-8 A8 4.2 starts at $78,050. Standard features include heated leather upholstery, power front seats, an iPod-compatible Bose stereo, a moonroof and a navigation system. Options include a panoramic moonroof, ventilated seats, front and rear massagers and much more.

The long-wheelbase A8 L starts at $84,000. The W-12 A8, which comes only in extended-wheelbase form, hasn’t been priced yet, but given the outgoing W-12 A8’s pricing, expect it to start at six figures. A performance-oriented S8 is likely to join the lineup down the road, too.

A8 in the MarketIn terms of sales, the old A8 was little more than a bit player among top-shelf luxury sedans. Its successor shouldn’t have much trouble improving on that, but I’m skeptical about its ability to attain the popularity of its BMW and Mercedes rivals. Both those cars exemplify the basic tenets of a large luxury sedan: spacious comfort and effortless capability. The A8 didn’t nail the basics — a shame, given its other strengths could have given Audi some inroads among top-end luxury buyers.